The present invention relates to apparatus for low-pressure deposition of materials, and more particularly to apparatus for sputter coating, and most particularly to a conical target and associated magnetron apparatus for making sputtered coatings having extremely high thickness uniformity.
Thickness uniformity requirements for sputtered coatings are becoming increasingly stringent. For example, the so-called LO layer in digital video disks must have a reflectivity variation of less than +/xe2x88x925% over the disk surface, requiring an equivalent uniformity in sputtered layer thickness. In some applications, only a minimum coated thickness or reflectivity is required, which may be readily achieved by coating to excess. However, this can shorten the useful lifetime of the sputtering target, and in the case of precious metals such as gold or silver such wasteful coating can be very expensive.
It is known in the art to move or rotate a substrate during coating to reduce areal variation in coated thickness, but newer generations of apparatus, such as cluster tools and optical disk coaters, typically coat a single substrate at a time without rotation. In some applications, the coating exposure time is less than one second. This means that the traditional method of moving the substrate relative to the source to produce more uniform coatings is either not possible or not practical.
One way of achieving good uniformity when source and substrate are fixed with respect to each other is to use diode sputtering. This process removes material uniformly from a planar target and deposits it efficiently on a closely spaced parallel substrate. FIG. 1 is a graph of calculations based on a uniform cosine distribution of material from each elemental area of a planar target 21 cm in diameter. This diameter was chosen to illustrate the possibility of good uniformity on a substrate 12 cm in diameter, such as a digital videodisk. FIG. 1 shows that a thickness uniformity of approximately +/xe2x88x921% can be achieved through diode sputtering.
Even though diode sputtering can result in good uniformity on circular substrates, it is almost never used because of several serious drawbacks. First, it produces relatively low sputtering rates at reasonable power densities, which translates into long coating times and low throughput. Second, diode sputtering requires high sputtering pressures, which results in less desirable film properties. Third, the diode sputtering process generates electrons, which are accelerated away from the target at high energy which can damage or excessively heat the substrate being coated.
Magnetron sputtering overcomes these limitations such that virtually all modern sputtering is done with magnetron cathodes. These devices use magnetic fields to confine electrons to the vicinity of the target surface, resulting in more efficient use of electrons and higher plasma densities. This translates into lower operating pressures, less electron bombardment of the substrate, and higher deposition rates.
In a magnetron cathode, the erosion rate is highest where the lines of magnetic induction (magnetic field lines) are parallel to the target surface. Therefore, in order to use as much of the target as possible and to maximize the uniformity of deposition as well, one useful design confines the electrons with a combination of a magnetic field and electric field, known in the art as a hollow cathode configuration. An example of one such design is disclosed in U.S. Pat. No. 4,486,287.
A disadvantage of all planar magnetrons, in which the target is essentially a flat surface, is that the magnetic field lines must pass through the plane of the target in the inner portion of the target. This makes it impossible to magnetron sputter the surface of a planar target uniformly over its entire area, since there is always a central portion from which no sputtering takes place (whether or not target material is located there). The consequence of this on film uniformity is significant and is highly undesirable. FIG. 2 shows the effect of eliminating a central portion 5 cm in diameter of the previously described 21 cm target, such as is the case in a typical planar magnetron. Even though this central portion represents a relatively small percentage of the target area, the optimum uniformity on a 12 cm substrate is significantly reduced as a result of not using the central portion of the target. Moving farther away can improve uniformity but at a severe cost in materials utilization and deposition rate.
Minimizing the extent of this central portion is the subject of U.S. Pat. No. 5,597,459. In this configuration, the size of the magnetic pole piece is minimized to reduce the non-sputtered area. However, as shown in FIG. 3, even if the non-sputtering central area is reduced to a diameter of only 3 cm., there is still a significant penalty in terms of uniformity loss.
Achieving high uniformity over stationary substrates, therefore, presents a substantial problem for planar magnetrons. Several patents (see, for example, U.S. Pat. Nos. 4,595,482; 4,606,806; and 4,810,3470) disclose the simultaneous use of two independent concentric targets to achieve better uniformity than a single target alone can produce. However, such designs are relatively complex and sometimes require independent control of power to each target. Moreover, they do not use the hollow cathode concept, which means that the targets will not erode uniformly over their surfaces, resulting in relatively poor target utilization, as well as a distribution of sputtered material which changes the areal uniformity over the lifetime of the target.
There is a class of magnetron sputtering cathodes, which provide the advantages of essentially uniform material erosion over virtually the entire target surface and a stable coating profile over the entire target lifetime. These devices, known in the art as inverted cylindrical magnetrons, also use the hollow cathode confinement technique (see, for example, U.S. Pat. Nos. 3,884,793; 3,995,187; 4,030,996; 4,031,424; 4,041,353; 4,111,782; 4,116,793; 4,116,794; 4,132,612; and 4,132,613). However, instead of being a planar surface, the target typically is the inside surface of a cylinder. Such cathodes are available commercially for coating the outsides of wires, fibers, and three dimensional objects which are placed in or passed through the cylinder.
We have found, completely unexpectedly, that a flat, circular substrate placed with its surfaces normal to the axis of such a cylindrical magnetron and beyond the end of the cylinder can receive a relatively uniform coating (see the proceedings of the 39th Annual Technical Conference of the Society of Vacuum Coaters, 1996, p.97). For example, in FIG. 4 is shown a calculation of the coating profile produced by a cylindrical magnetron 10 cm long and 21 cm in diameter sputtering onto a flat substrate surface placed at three different distances from the end of the cylindrical target. Surprisingly, at a distance of 2.5 cm from the end, the uniformity variation is slightly better than +/xe2x88x921%. However, a disadvantage of using cylindrical magnetron sputtering to coat a flat substrate is that, unless substrates are placed at both ends, substantially half of the sputtered material is lost out the unused end of the cylinder.
It is a principal object of the invention to provide an improved apparatus for making highly uniform sputtered coatings on planar substrates.
It is a further object of the invention to provide apparatus for making sputtered coatings on planar substrates with reduced waste of target material.
It is a still further object of the invention to provide an improved hollow cathode magnetron, which sputters uniformly over its entire target surface.
It is a still further object of this invention to describe the conditions necessary to produce uniform erosion over the entire surface of a conical target.
Briefly described, a hollow cathode magnetron in accordance with the invention is provided in the shape of a truncated cone, also known as a conical frustum. The target cone is backed over its outer surface by a conical cathode maintained at a predetermined voltage for attracting gas ions into the inner surface of the target cone to sputter material therefrom. A combination of magnetic and electric fields, known as hollow cathode confinement, is used to produce a uniform sputtering plasma over virtually the entire surface of the conical frustum target. Sputter coatings on planar targets can achieve areal thickness nonuniformities of less than +/xe2x88x920.2%.